I'm not clear on what's going on here.
when you say vsc light stays on, do you mean it shows this "VSC Trac Off"? or something like that right? do you also see the X (for center diff) light on too or no?
does this light stay on even on hard surfaces too? it could be that the traction control is lit cuz it's slipping? not sure but if the vsc stays on while on the asphalt, then something doesn't sound right.
you do have the vsc button disengaged right? (have center unlocked)? just making sure since you don't have to drive w/ the center locked at all.
The codes should not clear just because you turn off your 4runner (or that's what I was led to believe.
As for the load, that shouldn't matter. My vehicle load dry weight is approximately 650# including the driver (The amount of metal I've added doesn't help me one bit). Add camping gear and approxmiately 25 gals of water and I'm talkin severely pushing the limits of the vehicle. Regardless I've never experienced anything of this kind though. sorry
It might be nothing but I'd check it out.